This invention relates to electronic test systems, and more particularly to margin testing of memory modules including SIMMs and DIMMs.
Testing of electronic circuits and systems is of critical importance. Electronic systems are usually mass-produced, and a small percentage of the systems produced are expected to fail. Testing ensures that those failing systems do not reach customers.
Electronic systems are described by various specifications that detail voltages to be applied to inputs, timing of signals, and temperatures of operation. Gross failures are quickly detected by a large power consumption or inability to generate expected outputs when a sequence of inputs is applied. While such major failures are easily detected, more subtle failures can also occur. For example, the system can meet all specifications at a nominal temperature, but at the maximum operating temperature it fails some of the timing requirements. A higher than normal resistance in an internal signal path could cause such a failure. A higher than normal resistance causes greater signal delay. At an elevated temperature, the resistance becomes even higher causing an even greater signal delay. This could result in a violation of timing requirements such as setup and hold times. For example, a greater than normal delay for a specific signal that is part of a data bus will have more skew relative to the other bits of the data bus. The system could operate within specifications when the typical Vcc power-supply voltage (Vcc) is applied, but fails some timing specifications when the minimum-specified Vcc is applied.
Electronic systems or parts that have such subtle failures are known as marginal parts, since they fail only at the margins or extremes of the specified operating conditions. Detecting such marginal parts is desirable, since such parts, if undetected, could be used in larger systems and cause these to fail. Automatic test equipment has been used to detect such failures, by applying varying voltages to the parts being tested. The temperature of the parts under test can also be adjusted by heating or cooling devices.
One of the most important of electronic parts is the dynamic-random-access memory (DRAM). DRAM memory chips are often mounted on small, removable memory modules. The original single-inline memory modules (SIMMs) have been replaced with dual-inline memory modules (DIMMs), and 184-pin RIMMs (Rambus inline memory modules) and 184-pin DDR (double data rate) DIMMs.
The memory-module industry is very cost sensitive. Testing costs are significant, especially for higher-density modules. Specialized, high-speed electronic test equipment is expensive, and the greater number of memory cells on high-speed memory modules increases the time spent on the tester, increasing costs.
Handlers for integrated circuits (ICs) have been used for many years in the semiconductor industry. Handlers accept a stack of IC chips that are fed, one at a time, to the tester. The tested IC is then sorted into a xe2x80x9cbinxe2x80x9d for IC chips that have passed or failed the test. More recently, handlers have been made for memory modules. U.S. Pat. No. 5,704,489 by Smith, describes in detail a xe2x80x9cSIMM/DIMM Board Handlerxe2x80x9d such as those in use today.
FIG. 1 shows a SIMM handler connected to a high-speed electronic tester. Memory modules 18 to be tested are loaded into the top of handler 10 in the input stack. Memory modules 18 drop down, one-by-one, into testing area. Module-under test MUT 20 is next to be tested. Arm 26 pushes MUT 20 laterally until it makes contact with contactor pins 16 that clamp down on xe2x80x9cleadlessxe2x80x9d connector pads formed on the substrate of MUT 20.
Contactor pins 16 are also connected to test head 14, which makes connection to tester 12. Tester 12 executes parametric and functional test programs that determine when MUT 20 falls within specified A.C. and D.C. parameters, and whether all memory bit locations can have both a zero and a one written and read back. Margin testing can be performed on some testers by varying voltages applied to different pins of the device being tested.
Tester 12 can cost from ten-thousand to millions of dollars. Cost can be reduced if a less-expensive tester replaces tester 12. Since most memory modules are intended for installation on personal computers (PCs), some manufacturers test memory modules simply by plugging them into SIMM or DIMM sockets on PC motherboards. A test program is then executed on the PC, testing the inserted module. Since PCs cost only about a thousand dollars, tester 12 and handler 10 of FIG. 1 are replaced by a lowcost PC. Equipment costs are thus reduced by a factor of a hundred.
FIG. 2 shows a PC motherboard being used to manually test memory modules. Substrate 30 is a motherboard. Components 42, 44, mounted on the top side of substrate 30, include ICs such as a microprocessor, logic chips, buffers, and peripheral controllers. Sockets for expansion cards 46 are also mounted onto the top or component side of substrate 30.
Memory modules 36 are SIMM or DIMM modules that fit into SIMM/DIMM sockets 38. SIMM/DIMM sockets 38 (hereinafter SIMM sockets 38) have metal pins that fit through holes in substrate 30. These pins are soldered to solder-side 34 of substrate 30 to rigidly attach SIMM sockets to the PC motherboard. Both electrical connection and mechanical support are provided by SIMM sockets 38.
Margin Conditions Would Cause PC Motherboard to Fail First
While using PC motherboards for testing memory modules greatly reduces equipment costs, margin testing is not performed. The SIMM sockets are integral with the substrate 30 of the PC motherboard, preventing variation of voltages applied to a memory module being tested in one of the sockets 38. The power-supply voltage (Vcc) to the entire PC motherboard could be varied, causing the Vcc to the memory module under test in socket 38 to also be varied. However, since the PC motherboard has so many components, increasing the power-supply voltage to the PC motherboard would likely cause failures in the motherboard components before failures occurred in the memory module being tested.
Likewise, hot air could be blown on the memory module being tested in socket 38. While this hot air would heat the module under test, it would also heat the PC motherboard and its components near socket 38, perhaps heating all of the motherboard to some extent. This heating is likely to cause failures of components 42, 44, or of solder and wiring connections, before the memory module fails. Thus margin testing of a memory module being tested in socket 38 is problematic.
The parent application teaches a small daughter card known as a test adapter board that is attached to the reverse side of the PC motherboard. The reverse-side attachment of the test adapter board facilitates attachment of the SIMM/DIMM handler, since the front side of the PC motherboard is too crowded for attaching the handler. The inventors realized that the back or solder-side of the PC motherboard is less crowded and provides unobstructed access.
The PC motherboard is modified to provide reverse attachment of the handler to the solder-side of the PC motherboard using the handler adapter board. The SIMM socket on the component side of the PC motherboard is removed, and the handler adapter board is plugged from the backside into the holes on the PC motherboard for the SIMM socket.
Handler Mounted Close to PC Motherboardxe2x80x94FIG. 3
FIG. 3 shows a SIMM/DIMM handler mounted close to the backside of the PC motherboard using the handler adaptor board. Handler 10 is not drawn to scale since it is several times larger than a PC motherboard. However, FIG. 3 does highlight how handler 10 can fit close to the removed SIMM socket. Such close mounting reduces loading and facilitates high-speed testing.
Contactor pins 16 within handler 10 clamp down onto leadless pads on the edge of module-under-test MUT 20 when arm 26 pushes MUT 20 into place for testing. Contactor pins 16 are electrically connected to connectors on the backside of handler 10. These connectors are edge-type connectors that normally connect with high-speed testers. Typically two connectors are provided. These male-type connectors fit into female-type connectors 54 mounted on handler adaptor board 50. Handler adaptor board 50 contains metal wiring traces formed therein that route signals from connectors 54 to adaptor pins 52 that protrude out the other side of handler adaptor board 50.
Adaptor pins 52 can be plugged into female pins 57 that are soldered onto solder-side 34 of the PC motherboard. Female pins 57 have extensions that fit into the through-holes exposed by removal of the SIMM socket, but also have cup-like receptacles for receiving adaptor pins 52. Using female pins 57 allows handler adaptor board 50 to be easily removed from substrate 30.
Once MUT 20 has been tested by a test program running on the PC motherboard, MUT 20 is sorted and drops down into either good bin 22 or bad bin 24. Sorting is in response to a pass/fail signal from the test program running on the PC motherboard.
Handler adaptor board 50 provides electrical connection from the module-under-test (MUT) in handler 10 to the removed SIMM socket on the PC motherboard. Handler adaptor board 50 provides a slight spacing or offset from the solder-side 34 surface of substrate 30, allowing handler 10 to be plugged directly into connectors 54 on handler adaptor board 50. Since the offset of adaptor board 50 is slight, the length of electrical connections to the handler is short, minimizing added loading on the PC""s memory bus. The relatively flat surface of solder-side 34 allows close mounting of the SIMM/DIMM handler to the PC motherboard.
Margin Testing Desirable
While the invention described in the parent application has been quite effective, further improvements are desired. In particular, margin testing of the memory module is desirable. A modification to the test adapter board that facilitates margin testing by varying voltages and signal timing applied to the memory module is desirable.
Reliability of the tested memory modules is improved with such margin testing. It would be desirable to vary voltages to only the memory modules being tested, while not varying voltages on the motherboard and to its components. Then failures that occurred are likely to be due to the memory module itself and not the motherboard.